This invention relates to a seat belt apparatus installed in a vehicle such as an automobile for protecting a passenger. More particularly, the invention relates to a seat belt retractor for locking a reel shaft, which takes up the seat belt, against rotation in the event of an emergency to prevent the seat belt from being pulled off the reel shaft.
In order to prevent a seat belt from being pulled off a reel shaft by inertial motion of a passenger in the event of an emergency such as when a large deceleration acts upon the vehicle, the conventional seat belt retractor installed in a vehicle such as an automobile is provided with locking means for locking the reel shaft, which takes up the seat belt, against rotation.
One example of such reel-shaft locking means is as disclosed in the specification and drawings of U.S. Pat. No. 4,796,918. Specifically, frame locking-type locking means is disclosed in which teeth are formed on a reel shaft and on a frame supporting the reel shaft. When deceleration greater than a predetermined value acts upon the vehicle, the reel shaft moves so that its teeth mesh with the teeth on the frame, thereby locking the reel shaft against rotation. This frame locking-type locking means can be formed so as to be comparatively light in weight because the teeth are formed on the frame and therefore a special member having such teeth is unnecessary. Accordingly, such locking means is fully capable of lending itself to a reduction in weight presently demanded in vehicles such as automobiles.
In a retractor equipped with such locking means, the teeth provided on the reel shaft and the teeth on the frame are each subjected to a comparatively large force when the teeth on the reel shaft mesh with the teeth on the frame. This makes it necessary to form these teeth to have a large width so as to mitigate the stress which they develop. Accordingly, a measure taken in the prior art to mitigate the stress developed by these teeth is to enlarge the thickness of the teeth on the reel shaft and the plate thickness of the frame so as to increase the meshing width of the teeth, or to reinforce the frame by attaching to it teeth formed on a separate member. However, a problem encountered with the former measure is an increase in weight. With the latter measure, the need for a reinforcing member not only results in a large number of parts but also necessitates steps for attaching the reinforcing member, thereby increasing the labor required and raising cost.
Furthermore, in the frame locking-type locking means described above, teeth provided on both ends of the reel shaft must be meshed simultaneously with teeth provided on both the right and left sides of the frame. In actuality, however, it is difficult to effect meshing simultaneously on both the right and left sides, and meshing often takes place on one side only. In particular, since the teeth on the reel shaft and the teeth on the frame mesh owing to movement of the reel shaft, achieving simultaneous engagement on both the right and left sides is very difficult. In addition, in a case where the teeth mesh on one side only, stress concentrates in the meshing teeth and much greater strength is required as a result. The inevitable consequence is a retractor of large size and weight.